In industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining characteristic measured variables of media, for example, of liquids and/or gases, flowing in a process line, for example, a pipeline, often such measuring systems are used, which, by means of a measuring transducer of vibration-type and a thereto connected, most often in a separate electronics-housing accommodated, transmitter electronics, induce in the flowing medium reaction forces, for example, Coriolis forces, and, derived from these, repetitively produce measured values correspondingly representing at least one measured variable, for example, a mass flow rate, a density, a viscosity or some other process parameter. Such measuring systems—often formed by means of an in-line measuring device in compact construction with integrated measuring transducer, such as, for instance, a Coriolis, mass flow meter,—have been known for quite some time and have proved themselves suitable for industrial applications. Examples of such measuring systems with a measuring transducer of vibration-type, or individual components thereof, are described e.g. in EP-A 763 720, EP-A 462 711, EP-A 421 812, EP-A 1 248 084, WO-A 98/40702, WO-A 96/08697, WO-A 2010/059157, WO-A 2008/059015, WO-A 2007/040468, WO-A 2005/050145, WO-A 2004/099735, U.S. Pat. No. 7,610,795, U.S. Pat. No. 7,562,585, U.S. Pat. No. 7,421,350, U.S. Pat. No. 7,392,709, U.S. Pat. No. 7,350,421, U.S. Pat. No. 7,325,461, U.S. Pat. No. 7,127,952, U.S. Pat. No. 6,883,387, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,092,429, U.S. Pat. No. 5,969,264, U.S. Pat. No. 5,926,096, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,734,112, U.S. Pat. No. 5,610,342, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,050,439, U.S. Pat. No. 5,009,109, U.S. Pat. No. 4,879,911, U.S. Pat. No. 4,823,614, U.S. Pat. No. 4,801,897, U.S. Pat. No. 4,768,384, U.S. Pat. No. 4,738,144, U.S. Pat. No. 4,680,974, US-A 2006/0283264, US-A 2011/0265580, US-A 2011/0167907, US-A 2010/0251830, US-A 2010/0242623, US-A 2010/0050783, or the not yet published international patent application PCT/EP 2012/056102 of the assignee.
Measuring transducers disclosed therein comprise at least two, equally constructed, essentially straight or curved, e.g. U-, or V-shaped, measuring tubes accommodated in a measuring transducer housing and serving for conveying the medium, in given cases, an inhomogeneous, extremely hot or even very viscous medium. The at least two measuring tubes can, such as shown, for example, in the mentioned U.S. Pat. No. 5,734,112, U.S. Pat. No. 5,796,011 or US-A 2010/0242623, be inserted into the process line via a flow divider extending on the inlet side between the measuring tubes and an inlet-side, connecting flange as well as via a flow divider extending on the outlet side between the measuring tubes and an outlet-side, connecting flange to form a tube arrangement with flow paths connected in parallel with one another. The measuring tubes can, however, also, such as shown, for example, in the mentioned EP-A 421 812, EP-A 462 711, EP-A 763 720, be inserted into the process line via in- and outlet tube pieces to form a tube arrangement with a single traversing flow path. In measuring operation, the measuring tubes, then flowed through by the medium in parallel or serially, are caused to vibrate for the purpose of generating in the flowing medium oscillation forms influenced by the flowing medium.
Selected as excited oscillation form—the so-called wanted mode—is, in the case of measuring transducers with curved measuring tubes, usually that eigenoscillation form (eigenmode), in the case of which each of the measuring tubes moves at least partially at a natural resonant frequency (eigenfrequency) about an imaginary longitudinal axis of the measuring transducer as an end-clamped cantilever in a pendulum-like manner, whereby Coriolis forces are induced in the medium flowing through as a function of mass flow. These, in turn, lead to the fact that there are superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes, thus pendulum-like, cantilever oscillations, thereto equal frequency, bending oscillations according to at least one likewise natural, second oscillation form of, in comparison to the wanted mode, higher (modal) order, the so-called Coriolis mode. In the case of measuring transducers with curved measuring tube, these cantilever oscillations in the Coriolis mode, forced by Coriolis forces, usually correspond to that eigenoscillation form, in the case of which the measuring tube also executes rotary oscillations about an imaginary vertical axis perpendicular to the longitudinal axis. In the case of measuring transducers with straight measuring tubes, in contrast, for the purpose of producing mass flow dependent, Coriolis forces, often a wanted mode is selected, in the case of which each of the measuring tubes executes, at least partially, bending oscillations essentially in a single imaginary plane of oscillation, so that the oscillations in the Coriolis mode are developed accordingly as bending oscillations of equal oscillation frequency and coplanar to the wanted mode oscillations.
For active exciting of oscillations of the at least two measuring tubes, measuring transducer of vibration-type have, additionally, an exciter mechanism driven during operation by an electrical driver signal, e.g. a controlled electrical current, generated and correspondingly conditioned by the mentioned transmitter electronics, respectively a therein correspondingly provided, special driver circuit. The exciter mechanism excites the measuring tubes to bending oscillations, especially opposite equal bending oscillations, in the wanted mode by means of at least one electro mechanical, especially electro dynamic, oscillation exciter flowed through during operation by the electrical current. The oscillation exciter acts practically directly, especially differentially, on the at least two measuring tubes. Furthermore, such measuring transducers include a sensor arrangement having oscillation sensors, especially electro dynamic, oscillation sensors, for the at least pointwise registering of inlet-side and outlet-side oscillations of at least one of the measuring tubes, especially opposite equal bending oscillations of the measuring tubes in the Coriolis mode, and for producing electrical sensor signals influenced by the process parameter (such as, for instance, the mass flow or the density) to be registered and serving as vibration signals of the measuring transducer. As described, for example, in U.S. Pat. No. 7,325,461, in the case of measuring transducers of the type being discussed, in given cases, the oscillation exciter can, at least at times, be used as oscillation sensor and/or an oscillation sensor at least at times as oscillation exciter. The exciter mechanism of measuring transducers of the type being discussed includes usually at least one electrodynamic oscillation exciter and/or an oscillation exciter acting differentially on the measuring tubes, while the sensor arrangement comprises an inlet-side, most often likewise electrodynamic, oscillation sensor, as well as at least one essentially equally-constructed, outlet-side, oscillation sensor. Such electrodynamic and/or differential, oscillation exciters of usually marketed measuring transducers of vibration-type are formed by means of a magnet coil affixed on one of the measuring tubes and flowed through, at least at times, by an electrical current—as well as by means of a rather elongated, especially rod-shaped, permanent magnet interacting with the at least one magnet coil, especially plunging into such, and serving as armature. The permanent magnet is correspondingly affixed on the other, opposite-equally moving, measuring tube. The permanent magnet and the magnet coil serving as exciter coil are, in such case, usually so oriented that they are essentially coaxial with one another. Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually embodied in such a manner and so placed in the measuring transducer that it acts, in each case, essentially centrally on the measuring tubes. In such case, the oscillation exciter and, insofar, the exciter mechanism, such as, for example, also shown in the proposed measuring transducers, is affixed to the particular measuring tube externally at least pointwise along an imaginary central, peripheral line of such measuring tube. Alternatively to an exciter mechanism formed by means of an oscillation exciter acting centrally and directly on the respective measuring tube, it is also possible, such as provided, among other things, in U.S. Pat. No. 6,092,429 or U.S. Pat. No. 4,823,614, to use, for example, exciter mechanisms formed by means of two oscillation exciters each affixed not in the center of the respective measuring tube to the measuring tube, but, instead, at the in- and outlet sides.
In the case of most marketed measuring transducers of vibration-type, the oscillation sensors of the sensor arrangement are of essentially the same construction as the at least one oscillation exciter, at least insofar as they work according to the same principle. Accordingly, also the oscillation sensors of such a sensor arrangement are most often formed, in each case, by means of: at least one coil affixed on one of the measuring tubes, wherein the coil is, at least at times, passed through by a variable magnetic field and, associated therewith, at least at times supplied with an induced measurement voltage; as well as a permanently magnetic armature affixed to another of the measuring tubes and interacting with the at least one coil for delivering the magnetic field. Each of the aforementioned coils is additionally connected with the mentioned transmitter electronics of the in-line measuring device by means of at least one pair of electrical connecting lines, which most often are led on as short as possible paths from the coils to the measuring transducer housing. Due to the superpositioning of the wanted- and Coriolis modes, the oscillations of the vibrating measuring tubes registered by means of the sensor arrangement on the inlet side and on the outlet side have a measurable phase difference dependent on the mass flow. Usually, the measuring tubes of such measuring transducers, e.g. applied in Coriolis, mass flow meters, are excited during operation to an instantaneous natural resonant frequency of the oscillation form selected for the wanted mode, e.g. at oscillation amplitude controlled to be constant. Since this resonant frequency is dependent especially also on the instantaneous density of the medium, market-usual Coriolis, mass flow meters can measure, besides mass flow, supplementally also the density of flowing media. Furthermore, it is also possible, such as shown, for example, in U.S. Pat. No. 6,651,513 or U.S. Pat. No. 7,080,564, directly to measure by means of measuring transducers of vibration-type viscosity of the medium flowing through, for example, based on an exciter energy required for maintaining the oscillations, or by means of the excitation power and/or based on an attenuation of oscillations of the at least one measuring tube resulting from a dissipation of oscillatory energy, especially that in the aforementioned wanted mode. Moreover, also other measured variables derived from the aforementioned primary measured values, mass flow rate, density and viscosity, such as, for instance, according to U.S. Pat. No. 6,513,393, the Reynolds number, can be ascertained.
In the case of measuring transducers of the type being discussed, it is of special importance to trim the oscillation characteristics of individual measuring transducer components, not least of all also of the at least one measuring tube, consequently the parameters characterizing, or influencing, said oscillation characteristics, such as, for instance, tube shapes, or -cross sections, tube wall thicknesses and, associated therewith, mass distributions, bending stiffnesses, eigenfrequencies etc., of each individual measuring transducer, for example, as exactly as possible, to a respectively nominal target measure therefor, namely to a target measure predetermined for defined reference conditions, or respectively to hold the scattering of said parameters within a population of produced measuring transducers of the same type in an as narrow as possible tolerance range predetermined therefor. Equally important in the case of measuring transducers of the type being discussed is to prevent, or correspondingly to minimize, possible imbalances of the respective tube arrangement brought about, for instance, by non-uniform, consequently non-symmetric, mass and/or stiffness distributions within the tube arrangement.
In such case, it is, among other things, also of special interest, at an as “late” as possible production phase, to adjust the eigenfrequencies of the respective tube arrangement of the measuring transducer to the desired target measure, here thus to one or more selected target eigenfrequencies, or correspondingly to compensate possible imbalances, in order to be able reliably to avoid possible recent detuning of the tube arrangement in a following production phase of the measuring transducer.
In the initially mentioned U.S. Pat. No. 5,610,342, for example, a method for the dynamic adjusting of a tube serving as measuring tube of a measuring transducer of vibration-type to a target stiffness is disclosed, in the case of which method the tube is pressed at its two tube ends in, in each case, into a bore of a first, respectively second, endpiece of a support tube by targeted plastic deformation of the tube walls in the region of the pipe ends and, as a result of such, the entire tube arrangement is adjusted to a target eigenfrequency. Furthermore, in the initially mentioned U.S. Pat. No. 7,610,795, a method is described for tuning a tube serving as measuring tube of a measuring transducer of vibration-type to a target eigenfrequency, consequently to a target bending stiffness co-determined by the tube geometry and cross section, by means of a fluid introduced therein and supplied with an (over-) pressure introducing plastic deformation of at least one part of its tube wall.
A disadvantage in the case of the methods known from the state of the art is, among other things, that they are very complicated. Moreover, another disadvantage of the aforementioned methods is that, as a matter of the principles involved, they lead ultimately to a certain change of the geometry of the tubes, namely to a deviation from the ideal circular shape of the cross section, respectively to an increased deviation from the perfect uniformity of the cross section in the longitudinal direction, and, consequently, to a deviation of the contour of the lumen of the tube from the ideal form.